Import MySQL remote sites into COW branches

[adamziel]

What it does

Allows forkpress remote clone ... --branch <branch> to work with normal MySQL-backed WordPress sites. When the synced remote cache does not include wp-content/database/.ht.sqlite, ForkPress exports the remote MySQL tables over SSH and imports them into the local SQLite sidecar before creating the COW branch.

Rationale

The previous path only worked for ForkPress-compatible remotes that already had .ht.sqlite. A standard WordPress host could sync files successfully, then fail during branch creation with:

source branch database does not exist: .../wp-content/database/.ht.sqlite

ForkPress branches need a local SQLite database so existing merge-base snapshots, row identity capture, and per-branch ID bands keep working. Importing MySQL into the cache keeps the rest of the branch/merge model unchanged.

Implementation

• Adds a remote mysqli exporter that reads common wp-config.php database constants without booting WordPress.
• Adds a local SQLite importer that creates tables, imports rows, preserves single-column autoincrement primary keys, and recreates secondary indexes.
• Runs the import automatically during remote clone when the cache has wp-config.php but no .ht.sqlite.
• Packages the new helpers in the production runtime bundle.
• Updates the remote-site docs to explain MySQL import requirements and troubleshooting.

Testing instructions

php -l scripts/cow/mysql_export.php
php -l scripts/cow/mysql_import_sqlite.php
php -l tests/cow/mysql_import.php
make test-cow-mysql-import
CARGO_TARGET_DIR=/tmp/forkpress-cargo-target FORKPRESS_RUNTIME_BUNDLE=/dev/null cargo test -p forkpress-cli
npm test
npm run build

I also ran the remote-cache e2e slice against a local debug binary built with a current temporary runtime bundle:

FORKPRESS_E2E_ONLY=remote-cache tests/cow/e2e.sh /tmp/forkpress-cargo-target-current/debug/forkpress